Miyako Matsui

Relationship of etch reaction and reactive species flux in C4F8/Ar/O2 plasma for SiO2 selective etching over Si and Si3N4

The relationship between reactive species flux and their modified surfaces was studied in a SiO2 highly selective etching over Si and Si3N4. Sample specimens with large patterns and φ 0.35 μm contact holes were etched using C4F8/Ar/O2 plasma in a dual-frequency (27/0.8 MHz) parallel-plate etching system. The amount of CFx reactive species was controlled by adjusting the C4F8 flow rate ratio while keeping the ion flux (3×1016 cm−2 s−1) and the Vpp of bias radio frequency (1450 V) constant. The highly selective etch process is attained in a certain condition of the radical flux. Quantitative analysis using x-ray photoelectron spectroscopy revealed that the etch rate strongly depended on the fluorocarbon (CF) film thickness formed during the etch reaction on SiO2, Si3N4, and Si. In the large-area-etching of Si and Si3N4, the CF film (< 2 nm) formed under conditions with low selectivity for SiO2 was thinner than the film (5–6 nm) formed in high-selectivity etch conditions. The CF film thickness on SiO2 were l...

ETCH RATE CONTROL IN A 27 MHZ REACTIVE ION ETCHING SYSTEM FOR ULTRALARGE SCALE INTEGRATED CIRCUIT PROCESSING

The etch rates of SiO2, photoresist, Si, and SiN in a 27 MHz reactive ion etching system at constant ion flux of 6×1016 cm−2 s−1 and ion energy of 1450 V were studied. Typical incident flux densities of CF2 and CF+ were on the order of 1017 and 1016 cm−2 s−1, respectively. The SiO2 etch rate was determined by the balance of the energy supplied by the total ion flux and the amount of the C–F reactive species supplied by radicals and ions. When we roughly assumed the surface reaction probabilities of F, CF, CF2 and CF3 to be 0.1, 0.1, 0.1, and 0.5, the SiO2 etch rate could be expressed well as a function of the total number of F in the net radical fluxes. To clarify the dominant flux including radicals and ions, however, further research on surface reaction probabilities on the actual etched surface must be conducted because the incident fluxes strongly depend on these constants of the surface reaction probability. Lowering the total ion flux or ion energy decreased the etch rate of SiO2. A higher ion flux ...

Observation of surface reaction layers formed in highly selective SiO2 etching

We characterized the surface reaction layers formed by a fluorocarbon plasma for SiO2 selective etching over Si and Si3N4, in order to understand the etch mechanism and develop a process and tool for future ultralarge-scale integrated circuit processing. Specimens were etched using C4F8/Ar/O2 plasma in a dual-frequency (27/0.8 MHz) parallel-plate reactive ion etching system. The relationship between ion energy (assumed to be equal to the peak-to-peak voltage Vpp of the rf bias) and the thickness of the surface reaction layers was quantitatively analyzed using x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The fluorocarbon polymer layer and the SiFxOy layer on the substrates were observed. We found that the etch rate was strongly affected by the ion energy and the thickness of the fluorocarbon film on etched materials. In a highly selective etch process, the thickness of the fluorocarbon layer on the SiO2 surface was below 1 nm, while that on the Si3N4 and Si substrates ...

Analysis of SiO2-to-Si3N4 selectivity in reactive ion etching using additional O2 gas

The mechanism for control of the Si3N4 etch rate in a self-aligned contact process when C4F8/Ar/O2 gas is used was investigated. The Si3N4 surfaces after dry etching were analyzed by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. After dry etching, a polymer-like residue film less than 15 nm thick and an oxidized layer between the polymer layer and the Si3N4 surface were observed. In the CF polymer layer, the etching product of CN or CNO was characterized. CN or CNO components are widely distributed in the CF polymer layer, while the SiO2 layer is more localized on the Si3N4 surfaces. We found that the thicknesses of the CF polymer and SiO2 layer strongly depend on the Si3N4 etch rate, which is determined by the flow rate of the additional O2 gas. The thickness of the CF polymer, which was decreased from 10.1 to 4.1 nm, is considered to act as an inhibitor. The thickness of the SiO2 layer, which is easy to etch, was increased from 1.4 to 2.7 nm. The CF polymer thickne...

Low-energy Ion-scattering Spectroscopic Analysis of Structural Damage in Si Substrate under Ultrathin SiO2 after Gate Etching

The relationship between wafer-bias power during over-etching and structural damage induced in Si(100) substrate under ultrathin SiO2 layers by gate etching was examined by low-energy ion-scattering spectroscopy (LEIS) and high-resolution X-ray photoelectron spectroscopy. The crystal structure of the topmost surface of the Si substrate was modified when the residual SiO2 layer was 2.5 nm thick at a wafer-bias RF power of 70 W during over-etching. This structural damage was shown to be due to ions coming from plasma irradiation because the residual SiO2 thickness was close to the projected ion range at an ion energy of 310 eV (which was estimated from the Vpp of the bias voltage when the RF power was 70 W).

Detecting Defects in Cu Metallization Structures by Electron-Beam Wafer Inspection

A technique using an electron-beam to inspect wafers with Cu interconnects was developed. It can detect defects such as voids or incomplete contact failures with resistances of more than 10 7 Ω by utilizing the voltage contrast of a secondary electron image. The technique is performed by controlling the charging voltage of the interconnect while the electron-beam is irradiating the wafer. The charging voltage was found to depend on the conditions of the incident electron-beam, including the incident energy. To investigate the generation of voltage contrast in a Cu via chain, the electronic characteristics of detected defects were measured with probes. It was found that the resistance of a defect measured during electron irradiation and that measured with probes agree well. The contrast of a secondary electron image depends on the resistance of the differential voltage between the ends of a defect during electron-beam irradiation. To detect voids with high sensitivity, the differential voltage between the ends of a defect should be kept between 1 and 2 V, because the resistance of the defect suddenly decreases when the differential voltage is increased beyond a certain point.

Plasma-Wall Interactions in Dual Frequency Narrow-Gap Reactive Ion Etching System

The effects of plasma-wall interactions on the plasma chemistry in a dual frequency narrow-gap reactive ion etching (RIE) system is investigated as a function of electron density, residence time, and partial pressure of additive O2 gas. It is found that there is a critical point in the residence time, where the dissociation dominant region and the wall-interaction dominant region are separated. The net flux of chemically reactive species is estimated. The net flux of carbon-inclusive ions is of the order of 1016 cm-2s-1, and is larger than that of CFx radicals. Carbon-inclusive and silicon-inclusive ions dominate the total flux of chemically reactive species. The deposition rate of the fluorocarbon film is strongly dependent on the O2 partial pressure, and is controlled by the chemical etching with oxygen as well as the reduction of fluorocarbon radical density in the gas phase. Based on the estimation of the net flux of chemically reactive species and the effect of added oxygen, dominant chemical species that control the etching reaction in the RIE system are discussed.

In-situ After-treatment Using Low-energy Dry-etching with a CF4/O2 Gas Mixture to Remove Reactive Ion Etching Damage

We have investigated a new method of in-situ after-treatment that precisely and anisotropically removes dry-etching damage induced by reactive-ion etching (RIE) of SiO2, especially in self-aligned contact (SAC) processing, through, low-energy etching in the dry-etching chamber after the RIE. This in-situ after-treatment with a CF4/Ar gas mixture and with a CF4/O2 gas mixture were examined. The after-treatment with the CF4/O2 gas almost completely removed both the chemical damage and the crystalline damage. On the other hand, although RIE damage could be reduced by after-treatment with the CF4/Ar gas mixture, severe crystal damage remained. The after-treatment with the CF4/O2 gas mixture could precisely remove the surface damage and allowed us to control the surface roughness during removal of RIE damage.

Quantitative measurement of voltage contrast in SEM images for in-line resistance inspection of wafers manufactured for SRAM

An in-line inspection method for partial-electrical measurement of defect resistance, which is quantitatively estimated from the voltage contrast formed in an SEM image of an incomplete-contact defect, was developed. This inspection method was applied to wafers manufactured for an SRAM device. That is, the gray scales of the defect images captured on an SRAM plug pattern were quantitatively analyzed. Accordingly, the gray scales of defective plugs formed for shared contact patterns were classified as two levels. The higher contrasts, which were calculated from the grayscales of the darker defects, were about 100%; the lower contrasts, which were calculated from the grayscales of the other defects, were from 38% to 60%. The resistances of these defects were estimated from a calibration curve obtained from the grayscales of the SEM images and the resistances of deliberately formed failures on standard wafers for voltage-contrast estimation. The estimated resistances of the lower-contrast defects (with an accuracy of about an order of magnitude) agree well with the resistances measured by nano-prober. It is concluded that this in-line inspection method for partial-electrical measurement is a useful technique for defect classification based on defect resistance and defect mode.

Mechanism of highly selective SiO2 contact hole etching

To investigate the mechanism of highly selective SiO2 contact hole etching, we analysed surfaces exposed to C4F8/Ar/O2 plasma in a dual-frequency parallel-plate etching system. The thickness and composition of the fluorocarbon layer on large areas and at the bottom of contact holes were quantitatively analysed, in conjunction with the flux of CFx chemical species and the etch rates. In a highly selective etch processes, the thickness of the fluorocarbon layer on the SiO2 surface is less than 1 nm, while that on the Si surface is 4–6 nm. We found that the etch rate is strongly affected by the thickness of the fluorocarbon film on the SiO2 and Si surfaces at the bottom of the contact holes, as well as on the large areas. However, a small increase of C4F8 gas flow causes a larger increase in the fluorocarbon film thickness at an aspect ratio of 4 than that on the large flat area surface. Furthermore, we observed a slightly thinner and C-rich film at an aspect ratio of 10. This was probably caused by the decrease in the radical flux passing through the small hole. This aspect-ratio dependence is the cause that the process window for highly selective hole etching tends to be narrow.